Ensure BufWritePost,FileChangedShellPost always process buffer.
Ensure BufWritePost,FileChangedShellPost always process buffer.

Before this change the TabEnter/BufEnter logic was intertwined with
the logic for BufWritePost,FileChangedShellPost.

Separating the concerns fixes a minor bug where a TabEnter-BufEnter
combination led to the plugin not updating the diff the first time the
buffer was written.

airblade/vim-gitgutter